1. Field of the Invention
The present invention relates to an electrolytic capacitor using a valvular metal such as aluminum, tantalum, or the like, to a method for producing the electrolytic capacitor, and, particularly, to an electrolytic capacitor using an conducting polymer as the cathode and its production method.
2. Description of the Prior Art
Customary electrolytic capacitors using a valvular metal, e.g. aluminum, tantalum, are generally fabricated using a valvular metal porous body as the anode, an oxide of a valvular metal as the dielectric layer, and an liquid electrolyte or an inorganic solid electrolyte as the cathode. As the cathode, for example, an organic solvent including an organic acid or the like is used in an aluminum electrolytic capacitor and manganese dioxide or the like in a tantalum electrolytic capacitor. In addition to the above, lead portions connecting to the anode and the cathode and finally a casing are formed.
Excellent responsiveness at high frequencies has been demanded of electronic parts along with the digitization of circuits. It is therefore required for the electrolytic capacitor to be improved in the excellent responsiveness at high frequencies by a reduction in the resistance. In this situation, the use of a high conducting polymer as a cathode has been studied and developed.
The conducting polymers used for conventional electrolytic capacitors are produced by polymerizing corresponding monomers by an electrolytic oxidation polymerization method or a electrochemical oxidation polymerization method. In these conducting polymers, a dopant anion is coordinated to improve the electronic conductivity. The doping with the dopant is usually performed in a solution in which the dopant coexists with monomers when the monomer is polymerized.
As the dopant, various anions such as a chlorine ion, sulfuric ion, and organic acid ion are utilized. The organic acid-type anions having a molecular structure with a large steric hindrance to the free movement of the organic anion in a polymer are industrially used to restrain the dedoping of the dopant from the formed conducting polymer layer.
The electrolytic capacitor has the structural feature in which an oxide film is formed on the surface of a porous valvular metal and the surface of pores of the valvular metal and is used as a dielectric layer, and the valvular metal remaining inside the dielectric layer is used as a anode. When forming a cathode in the capacitor, it is therefore necessary to coat, with the conducting polymer, the entire surface of the valvular metal, specifically, extending from the open surface of the porous body to the surface of very intricate internal pores in an efficient manner. In the case of producing the conducting polymer layer by the electrochemically oxidizing polymerization method, though the conducting polymer is formed into a densified coating with a low resistance and high quality, only an insufficient coating of the conducting polymer is formed in the inside of the porous body, posing the problem that no original capacitance is created due to imperfect coating.
On the other hand, formation of the conducting polymer by the chemical oxidation polymerization method results in that a coating of the conducting polymer is entirely formed even on the surface of internal of the porous body with ease. The polymer, however, is basically formed as precipitates exhibiting poor density and adhesion to a dielectric layer as described in USP 4697001. Thus this method gives rise to the problem that no uniform conducting polymer layer with a sufficiently low resistance can be obtained in a stable manner.